Combined effects of arthropod herbivores and phytopathogens on plant performance

1. Many plants are simultaneously attacked by arthropod herbivores and phytopathogens.These may affect each other directly and indirectly, enhancing or reducing the amount of plantresources they each consume. Ultimately, this may reduce or enhance plant performance relativeto what should be expected from the added impacts of herbivore and pathogen when theyattack alone.2. Previous studies have suggested synergistic and antagonistic impacts on plant performancefrom certain combinations of arthropods and pathogens, for example, synergistic impacts fromnecrotrophic pathogens together with wounding arthropods because of facilitated infectionand antagonistic impacts from induction of pathogen resistance by sucking herbivores.3. We compiled published studies on the impact of plant–herbivore–pathogen interactions onplant performance and used meta-analysis to search for consistent patterns of impacts amongplant, herbivore and pathogen characteristics and experimental conditions, and to test thesuggested hypotheses on synergistic or antagonistic impacts.4. None of the hypotheses based on proximate interactions between arthropods and pathogenswere supported by our analysis; in contrast, the patterns we found were related to plant traitsand experimental conditions.5. Our results suggest that immediate loss of resources from interactions between arthropodherbivores and pathogens is generally moderated by compensation to an extent where thereare no interactive effects on plant performance. However, as interactive impacts also differedamong environments and parasite manipulation methods, this suggests that the ability ofplants to compensate such losses may depend on environmental conditions and probably alsooverall infection load.